In many silver imaging systems, image density is provided by silver itself. In view of the increasing cost of silver, it is important to reduce both the amount of silver in the emulsion and the amount of silver remaining in the image. One measure of the ability of silver within the emulsion to provide image density is referred to as covering power. This, as is well-known in the art, is defined as the maximum optical density obtainable for a given coating weight of silver, or more specifically, ##EQU1## The goal in silver containing imaging systems is to use less silver to produce the desired maximum optical density.
Previous attempts to improve covering power have involved use of certain additives in silver halide emulsions. U.K. Patent Specification No. 1,019,693 teaches the use of starch derivatives for this purpose. U.K. Patent Specification No. 1,013,905 discloses use of a copolymer of acrylic acid and an N-substituted acrylamide to achieve an increase in covering power. Polyvinyl alcohols having molecular weights of 10,000 to 30,000 are disclosed in U.K. Patent Specification No. 1,062,933 to be useful in increasing the covering power of silver halide emulsions having a silver halide grain size predominantly in the range of 0.5-2 microns. A difficulty encountered with many additives aimed at increasing covering power is that they have an adverse effect on the hardness of the emulsion layer, with resultant deterioration in the physical properties of the film.
Increased sensitivity of a silver imaging system can also be related to increased covering power. U.S. Pat. No. 3,650,759 teaches use of 1,2-glycols to achieve improved sensitivity of a photographic silver halide emulsion, without an attendant increase in fog.
Various alcohols and cyclohexanes have been used in the art as gelatin plasticizers to stabilize films against mechanical stress, for example, U.S. Pat. No. 3,042,524 (polyhydric alcohols such as 1,2,4-butanetriol), U.S. Pat. No. 3,520,694 (lower alkyl trimethylols), U.S. Pat. No. 3,640,721 (cyclohexanes), U.S. Pat. No. 2,960,404 (dihydroxy alkanes such as 2,2-dimethyl-1,3-propanediol and 2-methyl-2,4-pentanediol), and U.S. Pat. No. 2,904,434 (ethylene glycolates).
Due to the increasing cost of silver, there remains a need in the art to develop emulsions having superior silver covering power. There also is a need to reduce or prevent the generation of fog during the coating and fast drying of silver halide photographic emulsions.